\documentclass[12pt]{article}
\usepackage{tikz}
\usetikzlibrary{automata, positioning, arrows.meta} % 添加了 arrows.meta

\begin{document}

\begin{center}
\begin{tikzpicture}[shorten >=1pt,node distance=2.5cm,on grid,auto, >= {Stealth}] % 使用 Stealth（注意大小写）

  \node[state, initial] (q_start)                    {start};
  \node[state]          (q_identifier_start) [right=of q_start] {identifier_start};
  \node[state, accepting] (q_identifier_body) [right=of q_identifier_start] {identifier_body};

  \path[->]
    (q_start) edge node {a-z, A-Z, \_} (q_identifier_start)
    (q_identifier_start) edge node {a-z, A-Z, \_, 0-9} (q_identifier_body)
    (q_identifier_body) edge [loop above] node {a-z, A-Z, \_, 0-9} {};

\end{tikzpicture}
\end{center}

\end{document}